The Richard Whitehead Young Papers at the UofU would give you some perspective from a military participant. RWY wrote a lot of articles for the Contributor concerning Mormons and military events — I don’t recall whether any of them dealt with the Spanish-American War, but it’s worth checking.

You should also read Conference talks for the period, especially the one where Brigham Young, Jr. objected to Mormon involvement, and responses by others who supported it. Those talks are undoubtedly cited in the Walker and/or Quinn articles.

I have a book at home on minorities using the patriotism and military service as a way establishing their claim to be fully American, and Mormons in the Spanish-American war and WWI were one of the case studies, if I recall. I’ll take a look.